Many applications used lithium batteries as a power source. As is known in the art, lithium batteries include “CR type” (manganese dioxide) batteries and “BR type” (polycarbon monofluoride) batteries. Regardless of its specific capacity, an older lithium battery often has more trouble functioning at low temperatures than when the battery was new. This is because the closed loop current voltage of the battery decreases as a percent of the depth of discharge (% DOD) in the battery. The increase in % DOD reflects the increase of the internal impedance in the battery increases as the battery ages. As a result, the battery is able to provide less current at a given voltage than it did when it was new.
Lithium batteries are often used to power various automotive transmitter devices, such as remote keyless entry (RKE) systems and tire pressure monitoring system (TPMS) devices. RKE and TPMS transmitters usually use a phase locked loop (PLL) device as an RF transmission device. These and other devices require a certain current and voltage level to work properly. As is known in the art, PLL devices require a minimum voltage level (e.g., around 2.1V to 2.2V) before instability in the voltage controlled oscillator (VCO) of the PLL device causes it to lose its lock.
PLL devices tend to consume a large amount of current during RF transmission. As the battery ages, the increase in its internal impedance tends to decrease at lower temperatures. The amount of current the battery supplies may still remain at a desired level, but the voltage of the battery will drop due to the increased impedance. If the voltage across the battery drops below the minimum level required by the PLL device, the PLL will not operate, abruptly halting RF communication.
There is a desire for a system that can improve the performance of battery-powered devices at low temperatures.